1. Field of the Invention
The present invention relates to a technique for all-optical signal regeneration used to repeat or regenerate signal light transmitted in digital optical communication networks. More particularly, the invention relates to a method and a system for all-optical signal regeneration that eliminate various types of distortion and noise added to the pulsed signal light or optical signal pulses throughout transmission, thereby regenerating signal light pulses having the same waveform, intensity, and timing (or, the same waveform and intensity) as the incident or input signal light pulses at the transmission end through optical signal processing.
2. Description of the Related Art
With existing optical communications systems, it is popular to use the conventional repeating method that repeats the intensity of pulsed signal light during transmission with optical fiber amplifiers. As these optical fiber amplifiers, for example, Erbium-Doped Fiber Amplifiers (EDFAs) are typically used. This conventional repeating method has an advantage that data or signal transmission can be carried out without conversion between optical signals and electrical signals. Thus, this method is suitable to large-capacity optical communications systems.
With the conventional repeating method using optical fiber amplifiers, however, there is a disadvantage that the Signal to Noise Ratio (S/N) degrades as the transmission distance increases. This disadvantage is caused by the fact that noise induced by Amplified Spontaneous Emission (ASE) occurring in the amplifiers (i.e., ASE noise) tends to be superposed on the signal light transmitted. As a result, the possible, maximum transmission distance is limited.
To cope with the above-described disadvantage, in other words, to suppress effectively the S/N degradation due to the ASE noise, application of the “optical 3R repeating” or “optical 2R repeating” method has been studied so far.
The “optical 3R repeating” is a method of repeating a digital or pulsed signal light by regenerating pulsed light which is the same in waveform, intensity, and timing as the initial signal light at the transmitting end through optical signal processing. This method includes “Reamplifying”, “Reshaping”, and “Retiming” processes and therefore, it is termed the “optical 3R repeating” method. “Reamplifying” means the amplifying process of the signal waveform distorted, in other words, the raising process of the signal level. “Reshaping” means the shaping process of the signal waveform distorted to facilitate discrimination of the signal from noise. “Retiming” means the timing process of the signal waveform amplified and shaped to correct the timing of the signal pulses, and the control process of controlling the width and phase of the signal pulses.
On the other hand, the “optical 2R repeating” is a method of repeating a pulsed signal light by regenerating pulsed light which are the same in waveform and intensity as the pulsed initial light at the transmitting end through optical signal processing. Thus, the “optical 2R repeating” method is different from the “optical 3R repeating” method in that the “Retiming” process is not carried out.
With the “optical 3R repeating” method, optical clock (i.e., pulsed clock light) is extracted from the sequence of pulses of the signal light inputted and then, the clock light thus extracted and the signal light are subjected to the “AND” processing, thereby regenerating or repeating pulsed signal light. In this way, a similar signal processing to the conventional repeating method that includes optical to electrical conversion and/or electrical to optical conversion is performed for the signal light and the clock light at high speed with the use of known optical signal processing techniques.
On the other hand, with the “optical 2R repeating” method, the ASE noise, which is superposed on the signal light in the optical amplifier, is suppressed with optical noise-suppressing elements or devices. Thus, the level of S/N is returned to its initial one for repeating the signal light.
In recent years, various types of all-optical signal regenerator systems have been developed and disclosed to realize the “optical 3R or 2R repeating” method explained above.
For example, Nakamura et al. disclosed an optical 3R regenerator system in the Digest of Optical Amplifiers and their Applications, OAA 2000, Jul. 9–12, 2000, Quebec, Canada, PD4-1 to 4. This system includes symmetrical Mach-Zehnder-type optical switches, each of which is formed by combination of Semiconductor Optical Amplifiers (SOAs) and optical interferometers.
Billes et al. disclosed an optical 3R regenerator system in the Digest of 23rd European Conference on Optical Communications, EOOC '97, Sep. 22–25, 1997, Edinburgh, Scotland, Vol. 2, pp. 269–272. This system includes Mach-Zehnder-type interferometers with SOAs.
Philips et al. disclosed an optical 3R regenerator system of the symmetrical Mach-Zehnder-type comprising SOAs and optical interferometers in Electronics Letters, Vol. 34, No. 24, pp. 2340–2342, 1998.
Kelly et al. disclosed an optical 3R regenerator system of the symmetrical Mach-Zehnder-type comprising SOAs and optical interferometers in Electronics Letters, Vol. 35, No. 17, pp. 1477–1478, August 1999.
Ueno disclosed a DISC-type wavelength converter in IEEE Photonics Letters, No. 10, pp. 346–348, 1998. This converter serves as an optical 2R regenerator system.
Leuthold et al. disclosed a DISC-type wavelength converter in the Digest of Optical Amplifiers and their Applications, OAA 2000, Jul. 9–12, 2000, Quebec, Canada, QWD3-1, pp. 186–188. This converter serves as an optical 2R regenerator system.
All of the prior-art optical 3R and 2R regenerator or repeater systems described above have a function to remove the intensity noise of signal light (i.e., ASE noise superposed on the signal light in the optical fiber amplifier provided in the repeater in the transmission path). However, to exhibit the function of suppressing the intensity noise in the signal light, these prior-art systems must satisfy the condition or requirement that the magnitude of the nonlinear phase shift of the signal light, which is caused by the SOAs built in the systems, is equal to π (=180°). This is a known fact. For example, Electronics Letters, Vol. 32, No. 6, pp 566–567, March 1996 and IEEE Photonics Technology Letters, Vol. 10, No. 10, pp. 1413–1415 reported the same.
With the above-described prior-art 3R or 2R regenerator systems, when the bit rate (i.e., the repetition frequency) of the optical signal pulses inputted is very high, for example, the bit rate is 100 GHz or higher, the injection current of the SOA is set as much as approximately 100 mA, to 300 mA. In this case, the nonlinear phase shift applied to the optical signal pulses is approximately 0.3 π at most. For this reason, a problem that desired intensity-noise suppressing function is insufficiently exhibited will occur.
This problem can be solved if the injection current of the SOA is increased. In this case, however, there arises another problem that not only the power consumption of the SOA but also the power consumption of Peltier coolers for cooling the SOA themselves increase.
Moreover, with the above-described prior-art 3R or 2R regenerator systems, there is a problem that it is difficult to equalize the wavelength of the pulsed output light (i.e., the output light for regeneration or repeating) to that of the pulsed input signal light.